Light-emitting organic compound with improved reliability

ABSTRACT

By repeating a purification process of a light-emitting organic compound several times, a thin film made of the light-emitting organic compound to be used in an EL display device contains ionic impurities at the concentration of 0.1 ppm or lower and has a volume resistivity in the range of 3×10 10  Ωcm or larger. By using such a thin film as a light-emitting layer in the EL device, a current caused by reasons other than the carrier recombination can be prevented from flowing through the thin film, and deterioration caused by unnecessary heat generation can be suppressed. Accordingly, it is possible to obtain an EL display device with high reliability.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting organic compound(including a complex that contains a metal in its molecular formula)capable of providing Electro Luminescence (EL), and an EL display deviceutilizing the same. Typically, the present invention relates to ahigh-molecular type EL display device which utilizes a light-emittingorganic compound made of a high-molecular compound.

The present invention also relates to an electronic apparatus includingthe above-mentioned EL display device as its display portion. It shouldbe noted that the above-mentioned EL display device will be alsoreferred to as the OLED (Organic Light-emitting Diode).

2. Description of the Related Art

Development of a display device including an EL layer as a self-lightemitting element that utilizes EL phenomenon (i.e., the EL displaydevice) has been proceeded in these years. Since the EL display deviceis of the self-light emitting type, no back light is required to becontained therein, unlike a liquid crystal display device or the like.Moreover, the EL display device exhibits a wide viewing angle. From theabove features, the EL display device is advantageous to be used as adisplay portion for a portable device which is likely to be usedoutdoors.

A light-emitting layer as a principal portion of the EL element is madeof an insulating material. When a voltage is applied across a cathodeand an anode with the light-emitting layer interposed therebetween,carriers (electrons and holes) are injected into the light-emittinglayer and recombined to emit light. Thus, a current flowing through thelight-emitting layer is caused by the recombination of carriers. An ELmaterial that can be used for the EL display device is described in, forexample, Japanese Patent Application Laid-Open No. Hei 2-311591.

In a light-emitting element such as a light-emitting diode in which asemiconductor junction is formed, Na (sodium) that may exhibit anadverse effect as a movable ion causes a resistance value of thelight-emitting layer to decrease, and therefore, can cause a currentflow other than that caused by the carrier recombination. When such anunnecessary current flows, an amount of heat generation is increased andthe light-emitting layer is more likely to deteriorate. The samedisadvantage may occur in the EL device. However, any sufficientcountermeasure against the disadvantage caused by the movable ion hasnot been provided for the EL material.

SUMMARY OF THE INVENTION

The present invention is intended to provide an EL display device withhigh reliability. The present invention is also intended to provide anelectronic apparatus with a highly reliable display portion by utilizingsuch an EL display device as its display portion.

In accordance with the present invention, in order to prevent a currentfrom flowing due to reasons other than the carrier recombination, avolume resistivity of a thin film made of a light-emitting organiccompound in an EL device is set to be in the range of 3×10¹⁰ Ωcm orlarger. A volume resistivity of a thin film made of a light-emittingorganic compound in an EL device is set to be in the range from 1×10¹¹to 1×10¹² Ωcm (preferably, in the range from 1×10¹² to 1×10¹³ Ωcm). Inorder to obtain the volume resistivity value in the above range, theconcentration of ionic impurities contained in the thin film made of thelight-emitting organic compound is set to be equal to 0.1 ppm or lower(preferably, equal to 0.01 ppm or lower). The ionic impurity refers toan element belonging to Group I or II in the periodic table, andtypically to sodium (Na) or potassium (K).

Accordingly, in order to obtain the above-mentioned structure, it isnecessary to use such a light-emitting organic compound that containsionic impurities at the concentration of 0.1 ppm or lower (preferably,at the concentration of 0.01 ppm or lower).

In the case of sodium, the above-mentioned concentration range can becalculated to be 7×10¹⁷ atoms/cm³ or lower (preferably, 7×10¹⁶ atoms/cm³or lower). However, it is appropriate to consider that the totalconcentration of all of the ionic impurities should meet theabove-mentioned concentration range.

When a light-emitting organic compound made of a low-molecular compound(hereinafter, referred to as the low-molecular type EL compound) is usedfor obtaining the above-mentioned light-emitting organic compound, thelow-molecular type EL compound can be purified by a zone purificationmethod, a sublimation purification method, a recrystallization method, adistillation method, a filtration method, a column chromatographymethod, or a reprecipitation method.

On the other hand, when a light-emitting organic compound made of ahigh-molecular compound (hereinafter, referred to as the high-moleculartype EL compound) is used, values of molecular weight are likely to varyover a certain range since degree of polymerization cannot be completelycontrolled. Thus, a melting temperature of the resultant material cannotbe decided unambiguously at a certain value, and therefore, it becomesdifficult to perform purification. In this case, it is appropriate toperform a dialysis method or a high-performance liquid chromatographymethod. In particular, it is appropriate to perform an electrodialysismethod for efficiently eliminating ionic impurities in the dialysismethod.

In either of the above-mentioned purification methods, a purificationprocess is required to be repeated several times in order to reduce theconcentration of the ionic impurities to a level of 0.1 ppm or lower.More specifically, it is desirable to repeat a purification process atleast three times or more, and more preferably, five times or more.Instead of repeating the same purification process, it is of coursepossible to perform two or more different processes.

In the case where the filtration method is employed, it is preferable touse a filter provided with openings having a diameter of 0.1 μm (thisdiameter is particularly referred to as the diameter ofparticle-eliminating opening). Preferably, a filter with openings havinga diameter of 0.05 μm is used. A filter provided with openings having adiameter of 0.1 μm only allows particles having a diameter of 0.1 μm orsmaller to pass therethrough. Similarly, a filter provided with openingshaving a diameter of 0.05 μm only allows particles having a diameter of0.05 μm or smaller to pass therethrough.

As set forth above, in accordance with the present invention, alight-emitting organic compound containing ionic impurities at theconcentration of 0.1 ppm or lower (preferably, at the concentration of0.01 ppm or lower) is formed, and by using it, an EL device including athin film made of a light-emitting organic compound having a volumeresistivity in the range of 3×10¹⁰ Ωcm or larger. A volume resistivityof a thin film made of a light-emitting organic compound in an EL deviceis set to be in the range of 1×10¹¹ to 1×10¹² cm (preferably, in therange from 1×10¹² to 1×10¹³ Ωcm) is formed so as to fabricate an ELdisplay device by utilizing such an EL device.

For the light-emitting organic compound to be used in the presentinvention, as the low-molecular type EL compound, a compound having amolecular weight in the range of 1×10² to 8×10² g/mol (typically, in therange of 3×10² to 5×10² g/mol) can be used, while a compound having amolecular weight in the range of 8×10² to 2×10⁶ g/mol (typically, in therange of 1×10⁴ to 1×10⁵ g/mol) can be used as the high-molecular type ELcompound.

The typical low-molecular type EL compounds that can be used in thepresent invention include Alq₃ (tris-8-quinolinolato aluminum complex).Its molecular formula can be expressed as shown in Formula 1 below.

The other possible compounds include distyl allylene amine derivativethat can be obtained by adding amino-substituted DSA to DSA (distylallylene derivative). DSA can be expressed by Formula 2 below.

The typical high-molecular type EL compounds that can be used in thepresent invention include PPV (polyphenylenevinylene), which includesvarious types. For example, the molecular formulas 3 and 4, shown below,have been presented (in the article by H. Shenk, H. Becker, O. Gelsen,E. Kluge, W. Kreuder, and H. Spreitzer entitled “Polymers forLight-emitting Diodes” in Euro Display Proceedings 1999, pp.33-37).

Alternatively, polyphenylvinyl having a molecular formula as describedin Japanese Patent Application Laid-Open No. Hei 10-92576, as shown inFormulas 5 and 6, below, can also be used.

Various methods can be employed for forming a thin film of theabove-mentioned high-molecular type EL compounds. In particular, a spincoating method is preferred in view of simplicity in its process. Morespecifically, in the spin coating method, a solute which forms a thinfilm is dissolved in a solvent and the obtained solution is applied toan underlying member by means of a spinner or the like. Thereafter, thesolvent is volatilized in a baking process to form a thin film.

In accordance with the present invention, a solvent containing ahigh-molecular type EL compound is applied by means of a spinner, and aheat treatment is then performed at a temperature that is sufficientlylow for preventing crystallization of the high-molecular type ELcompound (specifically, at a glass-transition temperature or lower) soas to volatilize the solvent. As a result, a thin film made of thehigh-molecular type EL compound can be formed on the substrate.

Furthermore, since a light-emitting organic compound is vulnerable tooxygen, a conductive film to be formed following formation of the thinfilm made of the light-emitting organic compound is desirably formed insuch a condition that the thin film made of the light-emitting organiccompound is not exposed to surrounding atmosphere containing waterand/or oxygen. Accordingly, it can be preferable to form both the thinfilm made of the light-emitting organic compound and the conductive filmto function as a cathode or an anode in the same thin-film formationapparatus.

For meeting the above-mentioned requirement, a thin-film formationapparatus of the multi-chamber type is suitable. In the presentinvention, it is preferable to form an EL display device having highreliability by utilizing such a thin-film formation apparatus.

With the above-mentioned structure, a current that is caused by reasonsother than the carrier recombination can be prevented from flowingthrough a thin film made of the light-emitting organic compound that iscontained in an EL device, and deterioration caused by unnecessary heatgeneration can be prevented. Accordingly, it is possible to obtain an ELdisplay device with high reliability. Moreover, an electronic apparatuswith a highly reliable display portion can be obtained by utilizing suchan EL display device as its display portion.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a view for illustrating a pixel portion of an EL displaydevice;

FIGS. 2A through 2E are views for illustrating the fabricating steps ofan active matrix type EL display device;

FIGS. 3A through 3D are views for illustrating the fabricating steps ofan active matrix type EL display device;

FIGS. 4A through 4C are views for illustrating the fabricating steps ofan active matrix type EL display device;

FIG. 5 is a perspective view for illustrating the appearance of an ELdisplay device;

FIG. 6A is a top view for illustrating the appearance of the EL displaydevice;

FIG. 6B is a cross-sectional view for illustrating the structure of theEL display device;

FIG. 7 is a view for illustrating the cross-sectional configuration of apixel portion of the EL display device;

FIGS. 8A to 8F are views for respectively illustrating specific examplesof an electronic apparatus;

FIGS. 9A and 9B are views for respectively illustrating specificexamples of an electronic apparatus;

FIG. 10 is a diagram for illustrating the configuration of an apparatusto be used for forming a cathode layer, an EL layer, an anode layer andthe like; and

FIGS. 11A through 11F are views for respectively illustrating suitableshapes of a substrate fixing head to be used in a spin coating method.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One embodiment of the present invention will be described with referenceto FIG. 1. In FIG. 1, reference numeral 101 denotes a substrate havingan insulating surface. As the substrate 101, an insulating substratesuch as a quartz substrate can be used. Alternatively, various kinds ofsubstrate, such as a class substrate, a ceramic substrate, acrystallized glass substrate, a metal substrate (preferably a stainlesssubstrate), or a plastic substrate, can be used by providing aninsulating film on a surface thereof.

On the substrate 101, pixels 102 are formed. Although only three of thepixels are illustrated in FIG. 1, a higher number of pixels are actuallyarranged in matrix. For example, pixels are arranged in a matrix of640×480 for a VGA class, and in a matrix of 1024×768 for an XGA class.In each of the pixels 102, two TFTs are formed; one of them is aswitching TFT 103, and the other is a current-control TFT 104. A drainof the switching TFT 103 is electrically connected to a gate of thecurrent-control TFT 104. Furthermore, a drain of the current-control TFT104 is electrically connected to a pixel electrode 105 (which in thiscase, also functions as a cathode of an EL device). The pixel 102 isthus formed.

Respective wirings of the TFT as well as the pixel electrode can beformed of a metal film having a low resistivity. For example, analuminum alloy film may be used for this purpose. In addition, the TFTcan have any structure including a known structure.

Following the fabrication of the pixel electrode 105, a compound 106(which can be referred to as the cathode layer) that contains a metalhaving a low work function is formed over all of the pixel electrodes.It should be noted that the outline of the compound 106 is indicated bya dotted line in FIG. 1. This is because the compound 106 has athickness which is as thin as several nm, and it cannot be clearly knownwhether the compound 106 is formed as a layer or in an island-shape.

As a material for the above-mentioned compound 106 that contains a metalhaving a low work function, lithium fluoride (LiF), lithium oxide(Li₂O), barium fluoride (BaF₂), barium oxide (BaO), calcium fluoride(CaF₂), calcium oxide (CaO), strontium oxide (SrO), or cesium oxide(Cs₂O) can be used. Since these are insulating materials,short-circuiting between the pixel electrodes does not occur even whenthe compound 106 is formed as a layer.

It is of course possible to use a known conductive material such as aMgAg electrode instead of the above-mentioned compound 106. However, inthis case, the conductive material has to be selectively formed orpatterned into a certain shape in order to avoid short-circuitingbetween the pixel electrodes.

Over the compound 106 that contains a metal having a low work function,an EL layer 107 (a thin film made of a light-emitting organic compound)is formed. Although any known material and/or structure can be employedfor the EL layer 107, a material capable of emitting white light is usedin the present invention. With respect to the structure of the EL layer,only a light-emitting layer (a thin film made of a light-emittingorganic compound) for providing sites for the carrier recombination maybe included in the EL layer. Alternatively, if necessary, an electroninjection layer, an electron transport layer, a hole transport layer, anelectron blocking layer, a hole element layer, or a hole injection layermay be further layered to form the EL layer. In the presentspecification, all of those layers intended to realize injection,transport or recombination of carriers are collectively referred to asthe EL layer.

As a light-emitting organic compound to be used as the EL layer 107,either a low-molecular type organic compound or a polymer type(high-molecular type) organic compound can be used. However, it isdesirable to use a polymer type EL compound which can be formed by anformation technique that can be easily performed, such as a spin coatingmethod, a printing method, or the like. The structure illustrated inFIG. 1 is of the color display type in which an EL layer for emittingwhite light is combined with a color filter. Alternatively, a colordisplay scheme in which an EL layer for emitting blue or blue-greenlight is combined with fluorescent material (fluorescent colorconversion layer; CCM), or another color display scheme in which ELlayers respectively corresponding to RGB are overlaid each other toprovide a color display, can also be employed.

One of the features of the present invention is that a light-emittingorganic compound purified to an extremely high degree of purity is usedas a light-emitting layer. As a purification method, any known processfor purification can be used. For example, a zone purification method, asublimation purification method, a recrystallization method, adistillation method, a column chromatography method, or areprecipitation method can be used for a low-molecular type EL compound.For a high-molecular type EL compound, a dialysis method or ahigh-performance liquid chromatography method can be used. In the caseof the high-molecular type EL compound, it is also possible that apurification process similar to that for the low-molecular type ELcompound is performed prior to polymerization, and a compound is thenpolymerized.

In the case where a high-molecular type EL compound is to be purified bymeans of a dialysis method, an electrodialysis method is particularlypreferred for eliminating ionic impurities.

In the dialysis method, a polymerized high-molecular type EL material isput in a semi-permeable membrane made of a cellulose or the like, andimmersed in a solvent, e.g., pure water. The semi-permeable membrane isusually fixed by being supported between fine metal mesh partitions, orby being attached to a porous supporting member such as a circle platemade of half-melt glass. In the electrodialysis method, a voltage isapplied between metal mesh partitions supporting the semi-permeablemembrane, thereby resulting in a higher moving speed for ionicimpurities which can realize efficient purification.

In accordance with the present invention, the above-mentionedpurification process is repeated so that the concentration of ionicimpurities contained in the thin film made of the light-emitting organiccompound reaches a level of 0.1 ppm or lower (preferably, a level of0.01 ppm or lower). The above-mentioned concentration range of the ionicimpurities provides the thin film made of the light-emitting organiccompound, which functions as a light-emitting layer, with a volumeresistivity in the range of 3×10¹⁰ Ωcm or larger. A volume resistivityof a thin film made of a light-emitting organic compound in an EL deviceis set to be in the range of 1×10¹¹ to 1×10¹² cm (preferably, in therange from 1×10¹² to 1×10¹³ Ωcm). Thus, a current caused by reasonsother than the carrier recombination is prevented from flowing.

In the case where the EL layer includes only a light-emitting layer,i.e., only a single layer of a thin film made of the light-emittingorganic compound, the light-emitting layer is required to meetconditions in which the concentration of the contained ionic impuritiesis equal to or lower than 0.1 ppm (preferably, at 0.01 ppm or lower) andthe volume resistivity is in the range of 3×10¹⁰ Ωcm or larger. A volumeresistivity of a thin film made of a light-emittinq organic compound inan EL device is set to be in the range from 1×10¹¹ to 1×10¹² Ωcm(preferably, in the range from 1×10¹² to 1×10¹³ Ωcm).

It is of course critical to prevent ionic impurities from being mixedinto the light-emitting organic compound from the surrounding atmosphereduring a process step for purifying the light-emitting organic compoundwhich forms the EL layer and a process step for forming a film thereof.

Over the thus formed EL layer 107, a transparent conductive film isformed as an anode 108. As the transparent conductive film, a compoundof indium oxide and tin oxide (referred to as ITO), a compound of indiumoxide and zinc oxide, tin oxide (SnO₂), or zinc oxide (ZnO) can be used.

Over the anode 108, an insulating film as a passivation film 109 isprovided. As the passivation film 109, a silicon nitride film or asilicon nitride oxide film (represented as SiOxNy) is preferably used.

The substrate fabricated up to this stage is referred to as an activematrix substrate in the present specification. More specifically, thesubstrate on which a TFT, a pixel electrode electrically connected tothe TFT, and an EL device (a capacitor composed of a cathode, an ELlayer, and an anode) utilizing the pixel electrode as the cathode areformed is referred to as the active matrix substrate.

Furthermore, an opposing substrate 110 is attached to the active matrixsubstrate so that the EL devices are interposed therebetween. Theopposing substrate 110 is provided with light-shielding films 112 andcolor filters 113 a to 113 c. At this situation, each of thelight-shielding films 112 is provided so that a gap 111 formed betweenthe pixel electrodes 105 is unseen from the viewing direction of anobserver (i.e., from a direction normal to the opposing substrate). Morespecifically, each of the light-shielding films 112 is provided tooverlap (align with) the periphery of the pixel when viewed from thedirection normal to the opposing substrate. This is because this portionis non-emitting portion, and furthermore, electric field becomescomplicated at the edge portion of the pixel electrode and thus lightcannot be emitted from this portion with a desired luminance orchromaticity.

More specifically, by providing the light-shielding film 112 at theposition corresponding to the periphery (edge portion) of the pixelelectrode 105 and the gap 111, contour between the pixels can be madeclear. It can be also said that in the present invention, thelight-shielding film 112 is provided at the position corresponding tothe periphery (edge portion) of the pixel because the contour of thepixel electrode corresponds to the contour of the pixel. It should benoted that the position corresponding to the periphery of the pixelrefers to the position aligned with the periphery of the pixel whenviewed from the aforementioned direction which is normal to the opposingsubstrate.

Among the color filters 113 a to 113 c, the color filter 113 a is theone for obtaining red light, the color filter 113 b is the one forobtaining green light, and the color filter 113 c is the one forobtaining blue light. These color filters are formed at positionsrespectively corresponding to the different pixels 102, and thus,different color of light can be obtained for the respective pixels. Intheory, this is the same as the color display scheme in a liquid crystaldisplay device which uses color filters. It should be noted that theposition corresponding to the pixel refers to the position aligned withthe pixel when viewed from the direction which is normal to the opposingsubstrate. More specifically, the color filters 113 a to 113 c areprovided so as to overlap the pixels respectively corresponding theretowhen viewed from the direction normal to the opposing substrate.

It should be noted that the color filter is a filter for improving thecolor purity of light which has passed therethrough by extracting lightof a specific wavelength. Accordingly, in the case where the lightcomponent of the wavelength to be extracted are not many, there may bedisadvantages in which the light of that wavelength has an extremelysmall luminance or a deteriorated color purity. Thus, although nolimitation is imposed to an EL layer for emitting white light which canbe used in the present invention, it is preferable that the spectrum ofthe emitted white light includes emission spectrums of red, green andblue light components having purity of as high as possible.

The color filters 113 a to 113 c, similarly to the light-shielding films112, can contain a drying agent such as barium oxide. In this case, aresin film containing a drying agent and a pigment of red, green or bluecolor may be used as a color filter.

It should be noted that although not illustrated herein, the opposingsubstrate 110 is adhered to the active matrix substrate by means of asealing agent, so that a space designated with reference numeral 114 isa closed space. The closed space 114 may be filled with inert gas (noblegas or nitrogen gas), or with inert liquid. Alternatively, the closedspace 114 may be filled with a transparent adhesive so as to adhere thewhole surface of the substrate. Moreover, it is preferable to dispose adrying agent such as barium oxide in the closed space 114. Since the ELlayer 107 is very vulnerable to water, it is desirable to prevent waterfrom entering the closed space 114 as much as possible. Furthermore, itis advantageous to fill the closed space 114 with inert liquidcontaining crown ether or cryptand. Crown ether has an ability to trapsodium by being combined with them, and thus a gettering effect can beexpected to be realized.

As the opposing substrate 110, it is necessary to use a transparentsubstrate so as not to prevent light from traveling. For example, aglass substrate, a quartz substrate, or a plastic substrate ispreferably used. In addition, as the light-shielding film 112, a thinfilm capable of satisfactorily shielding light, e.g., a titanium film, aresin film including a black-colored pigment or carbon, can be used. Itis advantageous to use as the light-shielding film 112, a resincontaining a drying agent.

In the EL display device having the above-mentioned construction inaccordance with the present invention, the thin film made of thelight-emitting organic compound contains ionic impurities at theconcentration of 0.1 ppm or lower (preferably, at the concentration of0.01 ppm or lower) and a volume resistivity in the range of 3×10¹⁰ Ωcmor larger. A volume resistivity of a thin film made of a light-emittingorganic compound in an EL device is set to be in the range of 1×10¹¹ to1×10¹² Ωcm (preferably, in the range from 1×10¹² to 1×10¹³ Ωcm).Accordingly, a current caused by reasons other than the carrierrecombination can be prevented from flowing through a thin film made ofthe light-emitting organic compound that is contained in an EL device,and deterioration caused by unnecessary heat generation can beprevented.

Thus, it is possible to obtain an EL display device with highreliability. Moreover, an electronic apparatus with a highly reliabledisplay portion can be obtained by utilizing such an EL display deviceas its display portion.

In the EL display device in accordance with the present invention, lightemitted from the EL device passes through the opposing substrate to beemitted toward observer's eyes. Accordingly, the observer can recognizean image through the opposing substrate. In this situation, one of thefeatures of the EL display device in accordance with the presentinvention is that the light-shielding film 112 is disposed between theEL device and the observer so as to conceal the gap 111 between thepixel electrodes 105. Thus, the contour between the pixels can be madeclear, thereby resulting in an image display with high definition.

Furthermore, the light-shielding films 112 and the color filters 113 ato 113 c are disposed on the opposing substrate 110, and the opposingsubstrate 110 also functions as a ceiling material for suppressingdeterioration of the EL device. When the light-shielding films 112 andthe color filters 113 a to 113 c are disposed on the active matrixsubstrate, additional film-formation and patterning steps are required,thereby resulting in a reduced manufacturing yield. By providing thelight-shielding films 112 and the color filters 113 a to 113 c on theopposing substrate, reduction in the manufacturing yield can besuppressed.

Furthermore, the structure in accordance with the present invention, inwhich the opposing substrate 110 is provided with the light-shieldingfilms 112 and the color filters 113 a to 113 c and adhered to the activematrix substrate by means of the sealing agent, has features common tothe structure of a liquid crystal display device. Accordingly, it ispossible to fabricate the EL display device of the present inventionwith most of an existing manufacturing line for liquid crystal displaydevices. Thus, an amount of equipment investment can be significantlyreduced, thereby resulting in a reduction in the total manufacturingcost.

Embodiment 1

A first embodiment of the invention is described here. A descriptionwill be made here on a method for fabricating TFTs of a pixel portionand driving circuit portions provided around the same simultaneously.For simplicity of the description, only a CMOS circuit is shown which isa basic circuit for such driving circuits.

First, as shown in FIG. 2A, an base film 301 having a thickness of 300nm is formed on a glass substrate 300. In the present embodiment, the asilicon oxinitride film is laminated as the base film 302. At this time,the density of nitrogen in the region in contact with the glasssubstrate 300 is preferably in the range from 10 to 25 wt %.

Next, an amorphous silicon film (not shown) having a thickness of 50 nmis formed on the base film 301 using a known film forming method. Thefilm is not limited to an amorphous silicon film, and it may be anysemiconductor film (and any microcrystalline semiconductor film)including an amorphous structure. The film may alternatively be acompound semiconductor film including an amorphous structure such as anamorphous silicon germanium film. The thickness may be in the range from20 to 100 nm.

The amorphous silicon film is then crystallized using known techniquesto form a crystalline silicon film (also referred to “polycrystallinesilicon film or polysilicon film”) 302. Known methods forcrystallization include thermal crystallization utilizing anelectrically heated furnace, laser anneal crystallization utilizinglaser light and lamp anneal crystallization utilizing infrared light. Inthe present embodiment, crystallization is performed using excimer laserlight utilizing XeCl gas. While pulse-oscillated excimer laser lightformed in a linear configuration is used in the present embodiment, arectangular configuration may alternatively be used. Continuouslyoscillated argon laser light or continuously oscillated excimer laserlight may be used.

When Nd-YAG laser (wavelength 1.06 μm) is used, second harmonic or thirdharmonic is used and the illumination with a beam is carried out tocrystallize the above mentioned semiconductor film with 100 to 500 J/cm²energy density. The beam is formed in a linear or rectangularconfiguration by the optical light system.

In this embodiment, although the crystalline silicon film is used as theactive layer of the TFT, it is also possible to use an amorphous siliconfilm. Note that it is possible to form the active layer of the switchingTFT, in which there is a necessity to reduce the off current, by theamorphous silicon film, and to form the active layer of the currentcontrol TFT by the crystalline silicon film. Electric current flows withdifficulty in the amorphous silicon film because the carrier mobility islow, and the off current does not easily flow. In other words, the mostcan be made of the advantages of both the amorphous silicon film,through which current does not flow easily, and the crystalline siliconfilm, through which current easily flows.

Then, as shown in FIG. 2B, a protective film 303 constituted by asilicon oxide film is formed to a thickness of 130 nm on the crystallinesilicon film 302. A thickness within the range from 100 to 200 nm(preferably from 130 to 170 nm) may be chosen. Other types of insulationfilms may be used as long as silicon is included therein. The protectivefilm 303 is provided to prevent direct exposure of the crystallinesilicon film to plasma during doping with an impurity and to enabledelicate density control.

Resist masks 304 a through 304 b are formed on the protective film toallow doping with an impurity element that provides n-type conductivity(hereinafter referred to as “n-type impurity element”) through theprotective film 303. As the n-type impurity element, an elementbelonging to the group XV, typically, phosphorous or arsenic may beused. In the present embodiment, phosphorous is added in a density of1×10¹⁸ atoms/cm³ using a plasma doping process in which phosphine (PH₃)is plasma-excited without performing mass separation on the same. It isobviously possible to use an ion implantation process which involvesmass separation. The dose is adjusted such that n-type impurity regions305 and 306 formed at this step include the n-type impurity element in adensity in the range from 2×10¹⁶ to 5×10¹⁹ atoms/cm³ (typically, from5×10¹⁷ to 5×10¹⁸ atoms/cm³).

Next, as shown in FIG. 2C, the protective film 303 is removed toactivate the added element belonging to the group XV. While any knowntechnique may be used as means for activation, activation is carried outby means of illumination with excimer laser light. Obviously, theinvention is not limited to excimer laser light, and pulse-oscillated orcontinuously oscillated laser light may be used. Since the purpose is toactivate the added impurity element, illumination is preferably carriedout with an energy at which the crystalline silicon film is not melted.The illumination with laser light may be carried out with the protectivefilm 303 unremoved.

When the impurity element is activated with laser light, activation maybe simultaneously performed using furnace annealing. Referring toactivation using furnace annealing, a thermal process at a temperaturein the range from 450 to 550° C.

As a result of this step, the edges of the n-type impurity regions 305and 306, i.e., the boundaries (bonding portions) between the n-typeimpurity regions 305 and 306 and the regions around the same which arenot doped with the n-type impurity element becomes clear. Therefore,very preferable bonding portions can be formed between the LDD regionsand the channel forming region when the TFT is completed later.

Next, as shown in FIG. 2D, unnecessary portions of the crystallinesilicon film are removed to form island-shaped semiconductor films(hereinafter referred to as “active layers”) 307 through 310. Next, asshown in FIG. 2E, a gate insulation film 311 is formed to cover theactive layers 307 through 310. An insulation film including silicon witha thickness in the range from 10 to 200 nm (preferably in the range from50 to 150 nm) may be used as the gate insulation film 311. This film mayhave either of single-layer or multi-layer structures. In the presentembodiment, a 110 nm thick silicon oxinitride film is used.

Next, a conductive film having a thickness of 200 to 400 nm is formed,and patterning is carried out to form gate electrodes 312 to 316. Theend portions of the gate electrodes 312 to 316 can also be madetaper-shaped. Note that in this embodiment, the gate electrode and anextended wiring line (hereinafter referred to as a gate wiring line)electrically connected to the gate electrode are formed of differentmaterials. Specifically, a material having a resistance lower than thegate electrode is used for the gate wiring line. This is because amaterial which can be finely worked is used for the gate electrode and amaterial which has a low wiring resistance though fine working can notbe made is used for the gate wiring line. Of course, the gate electrodeand the gate wiring line may be formed of the same material.

While the gate electrode may be constituted by single-layer conductivefilms, multi-layer films such as double-layer or triple-layer structuresare preferably used as needed. Any known conductive film may be used asthe material for the gate electrodes. However, as described above, it ispreferable to use a material which can be finely worked, specifically,can be patterned into a line width of 2 μm or less. Specifically, it ispossible to use thin films including tantalum (Ta), titanium (Ti),molybdenum (Mo), tungsten (W), chromium (Cr) or conductive silicon (Si)or thin films which are nitrides of the same (typically tantalum nitridefilms, tungsten nitride films or titanium nitride films) or alloy filmswhich are combinations of the above elements (typically Mo—W alloys orMo—Ta alloys) or silicide films including the above elements (typicallytungsten silicide films or titanium silicide films). Such films may beused in either of single-layer and multi-layer structures.

In the present embodiment, multi-layer films formed by a 50 nm thicktantalum nitride (WN) film and 350 nm thick tungsten (W) film are used.They may be formed using a sputtering process. An inert gas such as Xe,Ne or the like may be used as the sputtering gas to prevent the filmsfrom coming off due to stress.

At this time, the gate electrodes 313 and 316 are formed such that theyoverlap a part of the n-type impurity regions 305 and 306 respectivelywith the gate insulation film 311 interposed. Such overlaps become LDDregions which overlap the gate electrodes later.

Next, as shown in FIG. 3A, an n-type impurity element (which isphosphorous in the present embodiment) is added in a self-aligningmanner using the gate electrodes 312 through 316 as masks. An adjustmentis performed such that resultant impurity regions 317 through 323 aredoped with phosphorous in a density in the range from 1/2 to 1/10(typically from 1/3 to 1/4) of that in the n-type impurity regions 305and 306. Specifically, a density in the range from 1×10¹⁶ to 5×10¹⁸atoms/cm³ (typically from 3×10¹⁷ to 3×10¹⁸ atoms/cm³ is preferable.

Next, as shown in FIG. 3B, resist masks 324 a through 324 c are formedto cover the gate electrodes and the like, and an n-type impurityelement (which is phosphorous in the present embodiment) is added toform impurity regions 325 through 331 heavily doped with phosphorous. Anion doping process utilizing phosphine (PH₃) is performed again, and thedensity of phosphorous in those regions is adjusted such that it iswithin the range from 1×10²⁰ to 1×10²¹ atoms/cm³ (typically from 2×10²⁰to 5×10²¹ atoms/cm³).

While this step forms the source regions or drain regions of then-channel type TFTs, a part of the n-type impurity regions 320 through322 formed at the step shown in FIG. 3A is left for the switching TFT.

Next, as shown in FIG. 3C, the resist masks 324 a through 324 c areremoved to form a new resist mask 332. A p-type impurity element (whichis boron in the present embodiment) is added to form impurity regions333 and 334 heavily doped with boron. An ion doping process utilizingdiborane (B₂H₆) is performed here to add boron in a density within therange from 3×10²⁰ to 3×10²¹ atoms/cm³ (typically from 5×10²⁰ to 1×10²¹atoms/cm³).

While the impurity regions 333 and 334 have already been doped withphosphorous in a density within the range from 1×10²⁰ to 1×10²¹atoms/cm³, boron is added here in a density which is at least threetimes the same density. As a result, the previously formed n-typeimpurity regions are completely inverted into the p-type to serve asp-type impurity regions.

The n-type or p-type impurity element added in the respective density isactivated after removing the resist mask 332. The means for annealingmay be furnace annealing, laser annealing or lamp annealing. In thepresent embodiment, a thermal process at 550° C. is performed for fourhours in a nitrogen atmosphere in an electrically heated furnace.

At this time, it is critical to eliminate oxygen from the surroundingatmosphere to a level of as low as possible. This is because when oxygenof even only a small amount exists, an exposed surface of the gateelectrode is oxidized, which results in an increased resistance andlater makes it difficult to form an ohmic contact with the gateelectrode. Accordingly, the oxygen concentration in the surroundingatmosphere for the above-mentioned activation process is set at 1 ppm orlower, preferably at 0.1 ppm or lower.

After the activation process is completed, the gate wiring 335 having athickness of 300 nm is formed. As a material for the gate wiring 335, ametal film containing aluminum (Al) or copper (Cu) as its main component(occupied 50 to 100% in the composition) can be used. The gate wiring335 is arranged so as to provide electrical connection for the gateelectrodes 314 and 315 of the switching TFT (see FIG. 3D).

The above-described structure can allow the wiring resistance of thegate wiring to be significantly reduced, and therefore, an image displayregion (pixel portion) with a large area can be formed. Morespecifically, the pixel structure in accordance with the presentembodiment is advantageous for realizing an EL display device having adisplay screen with a diagonal size of 10 inches or larger (or 30 inchesor larger).

Thereafter, a first interlayer insulating film 336 is formed, as shownin FIG. 4A. As the first insulating film 336, a single-layeredinsulating film containing silicon, or a layered film obtained throughcombination thereof, can be used. A thickness of the first insulatingfilm 336 can be set in the range from 400 nm to 1.5 μm. In the presentembodiment, the first interlayer insulating film 336 is formed to have alayered structure in which a silicon oxide film having thickness of 800nm is formed on a silicon nitride oxide film having a thickness of 200nm.

Furthermore, a heat treatment is performed in the atmosphere containinghydrogen of 3 to 100% at a temperature of 300 to 450° C. for 1 to 12hours so as to realize a hydrogenation treatment. In this treatment,dangling bonds in the semiconductor film are terminated with thermallyexcited hydrogens. As other processes for hydrogenation, a plasmahydrogenation process can be performed in which hydrogens generated byplasma are used. It should be noted that the hydrogenation process maybe performed during the formation of the first interlayer insulatingfilm 336. More specifically, the above-mentioned hydrogenation processcan be performed after forming a silicon nitride oxide film having athickness of 200 nm, followed by the formation of the remaining 800 nmportion of the silicon oxide film.

Then, contact holes are formed through the first interlayer insulatingfilm 336, and source wirings 337 to 340 and drain wirings 341 to 343 arethen formed. In the present embodiment, the electrode is formed as alayered film with a three-layered structure, including a 100 nm thick Tifilm, a 300 nm thick Al film containing Ti, and a 150 nm thick Ti film,that are continuously formed by a sputtering method. Other conductivefilms can be of course used.

Thereafter, a first passivation film 344 having a thickness in the rangeof 50 to 500 nm (typically, in the range of 200 to 300 nm) is formed. Inthe present embodiment, a silicon nitride oxide film having a thicknessof 300 nm is used as the first passivation film 344. Alternatively, asilicon nitride film may be instead used.

It is advantageous to perform a plasma treatment with a gas containinghydrogens such as H₂, NH₃, prior to the formation of the silicon nitrideoxide film. Hydrogens excited in the pre-process are supplied to thefirst interlayer insulating film 336. By performing the heat treatmentin such a situation, the film properties of the first passivation film344 are improved. Simultaneously, the hydrogens added to the firstinterlayer insulating film 336 are diffused downwards, thereby resultingin efficient hydrogenation of the active layer.

Then, a second interlayer insulating film 345 made of an organic resinis formed, as shown in FIG. 4B. As the organic resin, polyimide,polyamide, acrylic, BCB (benzocyclobutene) or the like can be used. Thesecond interlayer insulating film 345 is provided mainly forplanarization, and thus, acrylic capable of exhibiting satisfactoryplanarizing properties is preferred. In the present embodiment, anacrylic film is formed so as to have a thickness sufficient forrealizing the planarization of steps formed by the TFTs. Preferably, theacrylic film has a thickness in the range of 1 to 5 μm (more preferably,in the range of 2 to 4 μm).

Thereafter, a contact hole is formed in the second interlayer insulatingfilm 345 and the first passivation film 344 to reach the drain wiring343, and then the pixel electrode 346 is formed. In the presentembodiment, an aluminum alloy film (an aluminum film containing titaniumof 1 wt %) having a thickness of 300 nm is formed as the pixel electrode346. Reference numeral 347 denotes an end portion of the adjacent pixelelectrode.

FIG. 10 illustrates a thin-film formation apparatus to be used forcontinuously forming the EL layer and the anode layer. Morespecifically, FIG. 10 illustrates an apparatus to be used forcontinuously forming a transparent conductive film as the anode layer, ahigh-molecular type EL layer as the light-emitting layer, a metal filmcontaining an element belonging to Group I or II in the periodic tableas the cathode layer, and a silicon nitride film or a silicon nitrideoxide film as the second passivation layer.

In FIG. 10, reference numeral 401 denotes a transportation chamber inwhich transportation of a substrate into and out of the apparatus isperformed. The transportation chamber is also referred to as aload/unload chamber. A carrier 402 to which the substrate is mounted isplaced in the transportation chamber 401. Two of the transportationchambers 401 may be provided; one of them is used for transporting thesubstrate into the apparatus, while the other is for transporting thesubstrate out of the apparatus. Reference numeral 403 denotes a commonchamber provided with a mechanism 405 for transporting the substrate 404(hereinafter referred to as the transportation mechanism). Thetransportation mechanism 405 includes a robot arm or the like forhandling a substrate.

A plurality of process chambers (designated as 407 to 411, respectively)are coupled to the common chamber 403 via gates 406 a to 406 f. In theconfiguration shown in FIG. 10, the pressure in the common chamber 403is reduced to several mTorrs to several tens of mTorrs, and therespective process chambers are decoupled from the common chamber 403 bymeans of the gates 406 a to 406 f. In this case, the process chamber 408for solution application process is filled with inert gas so that theprocess is to be performed under a normal pressure. Accordingly, aprocess chamber 401 for vacuum evacuation is provided between the commonchamber 403 and the process chamber 408 for solution applicationprocess.

Accordingly, when the respective chambers are provided with anevacuation pump, respective processes can be performed in vacuum. As anevacuation pump, an oil rotation pump, a mechanical booster pump, aturbo molecular pump, or a cryopump can be used, and in particular, thecryopump is preferred since it is effective for eliminating water.

Further by referring to FIG. 10, reference numeral 407 denotes a processchamber for forming the cathode layer (hereinafter referred to as thethird film-formation process chamber). In this chamber 407, an auxiliaryelectrode for assisting the cathode is formed. A vapor deposition methodor a sputtering method is usually used, and among them, the vapordeposition method is more preferred since it introduces less damage to asubstrate to be processed. In either case, the third film-formationprocess chamber 407 is decoupled from the common chamber 403 by the gate406 b so that the film formation process can be performed in vacuum.

On the other hand, in the case where the vapor deposition method isperformed as a vapor-phase film-formation method, a vapor source has tobe provided. A metal film to be often used as the cathode layer is madeof an element belonging to Group I or II in the periodic table. However,this kind of metal film is likely to be oxidized, and therefore, it isdesirable to protect a surface thereof. In addition, the required filmthickness therefor is small. Thus, a conductive film having a lowresistivity is auxiliarily provided to reduce a resistance value of thecathode as well as to protect the cathode. As the conductive film havinga low resistivity, a metal film containing aluminum, copper or silver asits main component can be used. In the present embodiment, lithiumfluoride is used for an electron injection layer 348 shown in FIG. 4C,and the electron injection layer 348 is formed by a vacuum vapordeposition method to have a thickness of 5 nm.

By further referring to FIG. 10, reference numeral 408 denotes a processchamber for applying a solution containing the high-molecular type ELmaterial by a spin coating method (hereinafter referred to as thesolution application process chamber). As set forth above, since the ELmaterial is very vulnerable against water, the solution applicationprocess chamber 408 is required to be always held in inert atmosphere.

For transportation of a substrate, the pressure in the vacuum evacuationprocess chamber 412 is reduced to the same level as the common chamber403. Thereafter, the gate 406 d is opened under that condition, and asubstrate is transported. The gate 406 d is then closed, and the vacuumevacuation process chamber 412 is purged by inert gas to a normalpressure. Then, the gate 413 is opened at the time when the pressurereturns to the normal level, and the substrate is transported to thesolution application process chamber 408. This transportation may beperformed for every stage. Alternatively, the transportation may beperformed by means of specially-dedicated transportation means.

The solution application process chamber 408 is provided with a fixedhead for holding and rotating the substrate, and means for supplying asolvent containing a high-molecular type EL compound onto the substrateby an appropriate amount. The fixed head can be of the vacuum chuck typewhich has a simple configuration. However, the substrate may be deformedin a pattern corresponding to a shape of a suction port, resulting in adeviation in a thickness of the resultant EL layer. While the EL layeris to be formed to have a thickness in the range of 100 to 200 nm, thedeviation of the film thickness thereof is likely to lead todeteriorated display quality in which, e.g., the intensity of lightemission is varied.

FIGS. 11A through 11F respectively illustrate various configurations ofthe fixed head to be used for reducing such a deviation in the filmthickness to the minimum level. The suction port has a shape in whichconcentric grooves or a plurality of openings are provided. Evacuationto vacuum is performed through a coupling port provided beneath thesuction port so that a suction force is scattered two-dimensionally. Thefixed head with such a configuration is integrated with upper and lowerplates.

More specifically, FIG. 11A illustrates a top view of an upper plate1101 of the fixed head, and a plurality of openings 1103 are formed inconcentric patterns. FIG. 11B illustrates a lower plate 1102 providedwith an evacuation port 1105 combined with a cross-shaped common groove1104. FIG. 11C illustrates a cross-sectional view taken along line A-A′in FIGS. 11A and 11B in which the upper plate 1101 is overlaid the lowerplate 1102. FIG. 11D illustrates another example in which a plurality ofopenings 1108 are provided in an upper plate 1106 of the fixed head.FIG. 11E illustrates a lower plate 1107 provided with an evacuation port1110 combined with a circular-shaped common groove 1109. FIG. 11Fillustrates a cross-sectional view taken along line B-B′ in FIGS. 11Dand 11E in which the upper plate 1106 is overlaid the lower plate 1107.

In the present embodiment, PVK (polyvinylcarbazole), Bu-PBD(2-(4′-tert-butyl phenyl)-5-(4″-biphenyl)-1,3,4-oxydiazole), coumarin6,DCM1 (4-dicyanomethylene-2-methyl-6-p-dimethylaminostyryl-4H-pyran), TPB(tetraphenyl butadiene), or Nile Red is dissolved into1,2-dichloromethane or chloroform, and the resultant solution is appliedby a spin coating method. The number of revolutions is set in the rangefrom about 500 to 1000 rpm, and the spinning continues for 20 to 60seconds to obtain an uniformly applied film.

It should be noted that prior to the film formation of theabove-mentioned organic compound, the purification process (typically,the dialysis method) is repeated at least three times or more,preferably five times or more, so that the concentration of the ionicimpurities contained therein is reduced to 0.1 ppm or lower (preferably0.01 ppm or lower). Thus, the concentration of the ionic impuritiescontained in the light-emitting layer 349 shown in FIG. 4C is reduced to0.1 ppm or lower (preferably to 0.01 ppm or lower) and a volumeresistivity of the light-emitting layer 349 is set in the range of3×10¹⁰ Ωcm or larger. A volume resistivity of a thin film made of alight-emitting organic compound in an EL device is set to be in therange of 1×10¹¹ to 1×10¹² Ωcm (preferably, in the range from 1×10¹² to1×10¹³ Ωcm).

After completion of the solution application process, the gate 413 isopened and the substrate 412 is transported to the vacuum evacuationprocess chamber 412. After the gates 413 and 406 d are closed, vacuumevacuation is performed in such a condition. When the pressure in thevacuum evacuation process chamber 412 reaches the same reduced pressurecondition as the common chamber 403, the gate 406 d is opened so thatthe substrate is transported into the common chamber.

Although a baking process chamber 409 is provided in the illustratedconfiguration, it is possible to provide the vacuum evacuation processchamber 412 with a susceptor that can be heated so that a baking processcan be performed in the vacuum evacuation process chamber 412. When thebaking process is followed by vacuum evacuation, degassing can beprevented.

With further reference to FIG. 10, reference numeral 410 denotes aprocess chamber for forming the anode layer 350 (hereinafter referred toas the first film-formation process chamber). The vapor depositionmethod or the sputtering method can be preferably performed for the filmformation in this case. It should be noted that since the process isused for forming the anode layer on the light-emitting layer 349, theprocess is required not to damage the light-emitting layer 349. In thecase where the sputtering method is performed, a target made of thematerials as set forth above, such as ITO, a compound of indium oxideand zinc oxide, SnO₂, or ZnO, is used. The film is formed to have athickness of 30 to 300 nm.

Upon the film formation by the sputtering method, a surface on which thefilm is to be formed (i.e., the surface with the light-emitting layerformed thereon) may face upward (face-up type) or downward (face-downtype). In the case of the face-up type, the substrate transported fromthe common chamber 403 can be mounted onto the susceptor without beingrequired to change its orientation, thereby resulting in a simplifiedoperation. On the other hand, in the case of the face-down type, thetransportation mechanism 405 or the first vapor-phase film-formationprocess chamber 410 is required to be provided with a certain mechanismfor turning the substrate upside down, thereby resulting in acomplicated transportation mechanism. However, the face-down type has anadvantage in which a less amount of dust is attached to the substrate.

In the case where the vapor deposition process is performed in the firstfilm-formation process chamber 410, a vapor source is required to beprovided therein. A plurality of vapor sources may be provided. Itshould be also noted that a vapor source may be of the resistive-heatingtype or of the EB (electron beam) type.

With reference again to FIG. 10, reference numeral 411 denotes a processchamber for forming the second passivation film (hereinafter referred toas the second film-formation process chamber). As the second passivationfilm, a silicon nitride film or a silicon nitride oxide film is formedby a plasma CVD method. Accordingly, although not illustrated, a gassupply system for SiH₄, N₂O, NH₃ or the like, plasma generating meansutilizing an RF power source of 13.56 to 60 MHz, as well as substrateheating means are provided for the second film-formation processchamber. Since the EL layer is vulnerable to water or moisture, it ispreferable to form such a passivation film continuously after formingthe EL layer without allowing the EL layer to be exposed to thesurrounding atmosphere.

In the present embodiment, the layered structure including the electroninjection layer 348 and the light-emitting layer 349 as shown in FIG. 4Cis used as the EL layer. Alternatively, an electron transport layer, ahole transport layer, a hole injection layer, an electron blockinglayer, or a hole blocking layer can be further formed in the EL layer,if necessary.

The second passivation film 351 made of a silicon nitride film is formedby a plasma CVD method to have a thickness of 100 nm. This secondpassivation film 351 is intended to provide protection for thelight-emitting layer 349 against water or the like, and also function torelease heat generated in the light-emitting layer 349. In order tofurther enhance the heat radiation effect, it is advantageous to formthe second passivation film by forming a silicon nitride film and acarbon film (preferably a diamond-like carbon film) into the layeredstructure.

Thus, the active matrix type EL display device having the configurationas shown in FIG. 4C is completed. The active matrix type EL displaydevice in accordance with the present embodiment is provided with TFTshaving appropriate structures not only in a pixel portion but also in adriver circuit portion. Accordingly, the EL display device can exhibithigh reliability and improved operational characteristics.

As an n-channel TFT 205 in a CMOS circuit to be used for the drivercircuit, a TFT having the structure suitable for reducing the injectionof hot carriers so as not to decrease an operation speed is used. Thedriver circuit mentioned here includes a shift register, a buffer, alevel shifter, a sampling circuit (sampling and holding circuit) or thelike. A signal conversion circuit such as a D/A converter is alsoincluded therein for a digital driving.

In the present embodiment, as shown in FIG. 4C, an active layer of then-channel TFT 205 includes a source region 355, a drain region 356, anLDD region 357, and a channel-forming region 358, wherein the LDD region357 overlaps a gate electrode 313 via a gate insulating film 311interposed therebetween.

The LDD region is provided only on the side closer to the drain regionso as to prevent an operation speed from decreasing. An OFF currentvalue does not have a significant adverse effect in the case of then-channel TFT 205. Rather, it is more preferable to place an emphasisupon the operation speed. Accordingly, it is desirable to dispose theLDD region 357 so as to completely overlap the gate electrode therebyresulting in a reduced resistance value. In other words, it is desirablethat the LDD region is not offset with respect to the gate electrode.

Furthermore, deterioration of a p-channel TFT 206 in the CMOS circuitdue to the injection of hot carriers is almost negligible, and thus, itis not necessary to provide any LDD region for the p-channel TFT 206. Itis of course possible to provide the LDD region for the p-channel TFT206, similarly for the n-channel TFT 205, to exhibit countermeasureagainst the hot carriers.

In the actual process, after the structure shown in FIG. 4C iscompleted, the EL layer is sealed in the closed space by using theopposing substrate provided with the light-shielding films, aspreviously described with reference to FIG. 1. At this time, thereliability (lifetime) of the EL layer can be improved by setting aninert atmosphere within the closed space or disposing a moistureabsorbing material (e.g., barium oxide) in the closed space. Such asealing process of the EL layer can be performed by using the techniqueto be used in the cell assembly step for liquid crystal display devices.

After the sealing process of the EL layer is completed, a connector(flexible print circuit; FPC) is attached for connecting the terminalsextended from the elements or circuits formed on the substrate toexternal signal terminals, thereby completing a final product.

With now reference to the perspective view of FIG. 5, the structure ofthe active matrix type EL display device in accordance with the presentembodiment will be described. The active matrix type EL display deviceof the present embodiment includes a pixel portion 602, a gate drivercircuit 603, and a source driver circuit 604 provided on a glasssubstrate 601. Switching TFTs 605 in the pixel portion is of then-channel, and are respectively disposed at crossing points between gatewirings 606 connected to the gate driver circuit 603 and source wirings607 connected to the source driver circuit 604. A drain of each of theswitching TFTs 605 is connected to a gate of the correspondingcurrent-control TFT 608.

Furthermore, a source of each of the current-control TFTs 608 isconnected to a power supply line 609. In the structure in accordancewith the present embodiment, the power supply line 609 is provided witha predetermined voltage. In addition, the EL device 610 is connected toa drain of the corresponding current-control TFT 608. Since a cathode ofthe EL device 610 is connected to the drain of the current-control TFT608, it is desirable to use an n-channel TFT as the current-control TFT608.

The FPC 611 functioning as external input/output terminals is coupledwith connection wirings 612 and 613 for transmitting signals to thedriver circuits, and a connection wiring 614 that is in turn connectedto the power supply line 609.

Furthermore, the EL display device in accordance with the presentembodiment will be described with reference to FIGS. 6A and 6B. Asubstrate 1000 is an active matrix substrate. On the substrate, a pixelportion 1001, a source driver circuit 1002, and a gate driver circuit1003 are formed. Various wirings from the respective driver circuits areextended via connection wirings 612 to 614 to reach the FPC 611 and beconnected to an external device.

At this time, an opposing substrate 1004 is provided so as to surroundat least the pixel portion, and more preferably, both the drivercircuits and the pixel portion. The opposing substrate 1004 is adheredto the active matrix substrate 1000 by means of an adhesive (sealingagent) 1005 to form a closed space 1006 in cooperation with the activematrix substrate 1000. Thus, the EL device is completely sealed in theclosed space 1006 and shut out from the external air.

In the present embodiment, a photocurable epoxy-type resin is used asthe adhesive 1005. Alternatively, other adhesives such as an acrylatetype resin can be also used. A thermosetting resin can be also used ifacceptable in view of heat-resistance characteristics of the EL layer.It should be noted that the material is required to prevent oxygen andwater from passing therethrough as perfectly as possible. The adhesive1005 may be applied by a coating device such as a dispenser.

Furthermore, in the present embodiment, the closed space 1006 betweenthe opposing substrate 1004 and the active matrix substrate 1000 isfiled with nitrogen gas. Moreover, the opposing substrate 1004 isprovided on its inner side (on the side closer to the closed space) witha light-shielding film 1007 and a color filter 1008, as describedpreviously with reference to FIG. 1. In the present embodiment, a resinfilm containing barium oxide and a black-colored pigment is used as thelight-shielding film 1007, and a resin film containing a red-colored,green-colored, or blue-colored pigment can be used as the color filter1008.

Furthermore, as shown in FIG. 6B, the pixel portion is provided with aplurality of pixels each including an individually separated EL device.All of these El devices share an anode 1009 as a common electrode. TheEL layer may be provided only in the pixel portion, but is not requiredto be disposed over the driver circuits. In order to selectively providethe EL layer, a vapor deposition method employing a shadow mask, alift-off method, a dry etching method, or a laser scribing method can beused.

The anode 1009 is electrically connected to a connection wiring 1010.The connection wiring 1010 is a power supply line to be used forsupplying a predetermined voltage to the anode 1009, and is electricallyconnected to the FPC 611 via a conductive paste material 1011. Althoughonly the connection wiring 1010 is described herein, the otherconnection wirings 612 to 614 are also electrically connected to the FPC611 in the similar manner.

As described above, the structure as shown in FIGS. 6A and 6B candisplay an image on its pixel portion by connecting the FPC 611 toterminals of an external device. In the present specification, the ELdisplay device is defined as a module containing a product in which itbecomes possible to display an image when an FPC is attached thereto, inother words, a product obtained by attaching an active matrix substrateto an opposing substrate (including the one provided with an FPCattached thereto).

Embodiment 2

In the present embodiment, the case where the present invention isapplied to a simple-matrix type EL display device will be described withreference to FIG. 7. In FIG. 7, reference numeral 701 denotes a plasticsubstrate, 702 denotes a cathode made of a layered structure includingan aluminum film and a lithium fluoride film (more specifically, thelithium fluoride film is provided so as to be in contact with the ELlayer). In the present embodiment, the cathode 702 is formed by thevapor deposition method. Although not illustrated in FIG. 7, a pluralityof the cathodes are arranged in stripes along in a directionperpendicular to the drawing sheet.

On the cathode 702, an EL layer 703 (only including a light-emittinglayer) made of a high-molecular type EL compound is formed by theprinting method. In the present embodiment, PVK (polyvinylcarbazole),Bu-PBD (2-(4′-tert-butyl phenyl)-5-(4″-biphenyl)-1,3,4-oxydiazole),coumarin6, DCM1(4-dicyanomethylene-2-methyl-6-p-dimethylaminostiryl-4H-pyran), TPB(tetraphenyl butadiene), or Nile Red is dissolved into1,2-dichloromethane, and the resultant solution is transferred onto thecathode 702 by the printing method and then baked to form the EL layer703 for emitting white light.

It should be noted that prior to the film formation of theabove-mentioned organic compound, the purification process (typically,the dialysis method) is repeated at least three times or more,preferably five times or more, so that the concentration of the ionicimpurities contained in the high-molecular type EL compound is reducedto 0.1 ppm or lower (preferably, to 0.01 ppm or lower). Thus, theconcentration of the ionic impurities contained in the EL layer 703 isreduced to 0.1 ppm or lower (preferably, to 0.01 ppm or lower), and avolume resistivity of the EL layer 703 is set in the range of 3×10¹⁰ Ωcmor larger. A volume resistivity of a thin film made of a light-emittingorganic compound in an EL device is set to be in the range of 1×10¹¹ to1×10¹² Ωcm (preferably, in the range from 1×10¹² to 1×10¹³ Ωcm).

In the present embodiment, the single-layered structure including onlythe light-emitting layer is used for the EL layer 703. Alternatively, anelectron injection layer, an electron transport layer, a hole transportlayer, a hole injection layer, an electron blocking layer, or a holeelement layer can be further formed in the EL layer, if necessary.

After the formation of the EL layer 703, an anode 704 made of atransparent conductive film is formed. In this embodiment, a compound ofindium oxide and zinc oxide is formed as the transparent conductive filmby a vapor deposition method. Although not illustrated in FIG. 7, aplurality of the anodes are arranged in stripes with the longitudinaldirection thereof being perpendicular to the drawing sheet so as to beorthogonal to the cathodes. Furthermore, although not illustrated, inorder to apply a predetermined voltage to the anodes, wirings areextended from the respective anodes 704 to a portion to which the FPC isto be attached.

Following the formation of the anodes 704, a silicon nitride film isformed to have a thickness of 100 nm as the passivation film 705. Thispassivation film 705 functions as a protective film for preventing theEL layer 704 from being exposed to the surrounding atmosphere duringsuccessive attachment of a cover member or the like.

Thus, the EL devices are formed on the substrate 701. Thereafter, aplastic plate 706 is provided as the cover member 706, and thelight-shielding film 707 and the color filter 708 are formed on thesurface thereof. A resin containing carbon is used as thelight-shielding film 707, and resins respectively containing eitherred-color pigment, green-color pigment, or blue-color pigment, are usedas the respective color filters 708. These films can be formed by an inkjet method, the spin coating method, or the printing method.

In the structure in accordance with the present embodiment, lightemitted from the EL device passes through the cover member 706 to reachobserver's eyes, and therefore, the cover member 706 is transparent.Instead of a plastic plate used in the present embodiment, anytransparent substrate (or a transparent film) such as a glass plate, aPVF film can be used for the cover member 706.

After providing the cover member 706, the cover member 706 is attachedby means of a filler agent 710 (functioning as adhesives) to which adrying agent 709 is added. The attachment process can be performed byemploying a double-vacuum type attachment apparatus which is used forthe fabrication of solar cells. Thereafter, frame members 712 areattached by means of sealing members 711 made of a UV-curable resin. Inthe present embodiment, a stainless member is used as the frame members712. Finally, the FPC 713 is attached, thereby completing the EL displaydevice.

Embodiment 3

The EL display device fabricated in accordance with the presentinvention is of the self-emission type, and thus exhibits more excellentrecognizability of the displayed image in a light place as compared tothe liquid crystal display device. Furthermore, the EL display devicehas a wider viewing angle. Accordingly, the EL display device can beapplied to a display portion in various electronic devices. For example,in order to view a TV program or the like on a large-sized screen, theEL display device in accordance with the present invention can be usedas a display portion of an EL display (i.e., a display in which an ELdisplay device is installed into a frame) having a diagonal size of 30inches or larger (typically 40 inches or larger.)

The EL display includes all kinds of displays to be used for displayinginformation, such as a display for a personal computer, a display forreceiving a TV broadcasting program, a display for advertisementdisplay. Moreover, the EL display device in accordance with the presentinvention can be used as a display portion of other various electricdevices.

Such electronic devices include a video camera, a digital camera, agoggles-type display (head mount display), a car navigation system, asound reproduction device (an audio equipment), note-size personalcomputer, a game machine, a portable information terminal (a mobilecomputer, a portable telephone, a portable game machine, an electronicbook, or the like), an image reproduction apparatus including arecording medium (more specifically, an apparatus which can reproduce arecording medium such as a compact disc (CD), a laser disc (LD), adigital video disc (DVD), and includes a display for displaying thereproduced image), or the like. In particular, in the case of theportable information terminal, use of the EL display device ispreferable, since the portable information terminal that is likely to beviewed from a tilted direction is often required to have a wide viewingangle. FIGS. 8A to 8F respectively show various specific examples ofsuch electronic devices.

FIG. 8A illustrates an EL display which includes a frame 2001, a supporttable 2002, a display portion 2003, or the like. The present inventionis applicable to the display portion 2003. The EL display is of theself-emission type and therefore requires no back light. Thus, thedisplay portion thereof can have a thickness thinner than that of theliquid crystal display device.

FIG. 8B illustrates a video camera which includes a main body 2101, adisplay portion 2102, an audio input portion 2103, operation switches2104, a battery 2105, an image receiving portion 2106, or the like. TheEL display device in accordance with the present invention can be usedas the display portion 2102.

FIG. 8C illustrates a portion (the right-half piece) of an EL display ofhead mount type, which includes a main body 2201, signal cables 2202, ahead mount band 2203, a display portion 2204, an optical system 2205, anEL display device 2206, or the like. The present invention is applicableto the EL display device 2206.

FIG. 8D illustrates an Image reproduction apparatus including arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2301, a recording medium (a CD, an LD, a DVDor the like) 2302, operation switches 2303, a display portion (a) 2304,another display portion (b) 2305, or the like. The display portion (a)is used mainly for displaying image information, while the displayportion (b) is used mainly for displaying character information. The ELdisplay device in accordance with the present invention can be used asthese display portions (a) and (b). The image reproduction apparatusincluding a recording medium further includes a CD reproductionapparatus, a game machine or the like.

FIG. 8E illustrates a portable (mobile) computer which includes a mainbody 2401, a camera portion 2402, an image receiving portion 2403,operation switches 2404, a display portion 2405, or the like. The ELdisplay device in accordance with the present invention can be used asthe display portion 2405.

FIG. 8F illustrates a personal computer which includes a main body 2501,a frame 2502, a display portion 2503, a key board 2504, or the like. TheEL display device in accordance with the present invention can be usedas the display portion 2503.

When the brighter luminance of light emitted from the EL materialbecomes available in the future, the EL display device in accordancewith the present invention will be applicable to a front-type orrear-type projector in which light including output image information isenlarged by means of lenses or the like to be projected.

The aforementioned electronic devices are more likely to be used fordisplay information distributed through a telecommunication path such asInternet, a CATV (cable television system), and in particular likely todisplay moving picture information. The EL display device is suitablefor displaying moving pictures since the EL material can exhibit highresponse speed. However, if the contour between the pixels becomesunclear, the moving pictures as a whole cannot be clearly displayed.Since the EL display device in accordance with the present invention canmake the contour between the pixels clear, it is significantlyadvantageous to apply the EL display device of the present invention toa display portion of the electronic devices.

A portion of the EL display device that is emitting light consumespower, so it is desirable to display information in such a manner thatthe light emitting portion therein becomes as small as possible.Accordingly, when the EL display device is applied to a display portionwhich mainly displays character information, e.g., a display portion ofa portable information terminal, and more particular, a portabletelephone or a car audio equipment, it is desirable to drive the ELdisplay device so that the character information is formed by alight-emitting portion while a non-emission portion corresponds to thebackground.

With now reference to FIG. 9A, a portable telephone is illustrated,which includes a main body 2601, an audio output portion 2602, an audioinput portion 2603, a display portion 2604, operation switches 2605, andan antenna 2606. The EL display device in accordance with the presentinvention can be used as the display portion 2604. The display portion2604 can reduce power consumption of the portable telephone bydisplaying white-colored characters on a black-colored background.

FIG. 9B illustrates a sound reproduction device, a car audio equipmentin concrete term, which includes a main body 2701, a display portion2702, and operation switches 2703 and 2704. The EL display device inaccordance with the present invention can be used as the display portion2704. Although the car audio equipment of the mount type is shown in thepresent embodiment, the present invention is also applicable to an audioof the set type. The display portion 2702 can reduce power consumptionby displaying white-colored characters on a black-colored background,which is particularly advantageous for the audio of the portable type.

As set forth above, the present invention can be applied variously to awide range of electronic devices in all fields. The electronic device inthe present embodiment can be obtained by utilizing an EL display devicehaving the configuration in which the structures in Embodiments 1through 3 are freely combined.

As set forth above, in accordance with the present invention,deterioration of an EL device can be suppressed, resulting in improvedreliability of an EL display device. Furthermore, by using the ELdisplay device obtained in accordance with the present invention as adisplay portion of an electronic apparatus, reliability of the resultantelectronic apparatus can be improved.

1. A light-emitting device comprising: a substrate; a plurality of firstelectrodes arranged in a stripe shape over the substrate; alight-emitting layer over the plurality of first electrodes; a pluralityof second electrodes arranged in a stripe shape so as to intersect withthe plurality of first electrodes, over the light-emitting layer; aplurality of color filters over the plurality of second electrodes; anda cover member over the plurality of color filters, wherein thelight-emitting layer comprises total ionic impurities at theconcentration of 0.1 ppm or lower.
 2. A light emitting element accordingto claim 1, wherein the cover member is at least one selected from thegroup consisting of a plastic plate, a glass plate and a PVF film.
 3. Alight-emitting element according to claim 1, wherein the ionicimpurities are sodium or potassium.
 4. A light-emitting elementaccording to claim 1, wherein the plurality of first electrodes arecathodes.
 5. A light-emitting element according to claim 1, wherein theplurality of second electrodes are anodes.
 6. A light-emitting devicecomprising: a substrate; a plurality of first electrodes arranged in astripe shape over the substrate; a light-emitting layer over theplurality of first electrodes; a plurality of second electrodes arrangedin a stripe shape so as to intersect with the plurality of firstelectrodes, over the light-emitting layer; a plurality of color filtersover the plurality of second electrodes; and a cover member over theplurality of color filters, wherein the light-emitting layer comprisestotal ionic impurities at the concentration of 0.01 ppm or lower.
 7. Alight emitting element according to claim 6, wherein the cover member isat least one selected from the group consisting of a plastic plate, aglass plate and a PVF film.
 8. A light-emitting element according toclaim 6, wherein the ionic impurities are sodium or potassium.
 9. Alight-emitting element according to claim 6, wherein the plurality offirst electrodes are cathodes.
 10. A light-emitting element according toclaim 6, wherein the plurality of second electrodes are anodes.
 11. Alight-emitting device comprising: a substrate; a plurality of firstelectrodes arranged in a stripe shape over the substrate; alight-emitting layer over the plurality of first electrodes; a pluralityof second electrodes arranged in a stripe shape so as to intersect withthe plurality of first electrodes, over the light-emitting layer; aplurality of color filters over the plurality of second electrodes; anda cover member over the plurality of color filters, wherein thelight-emitting layer comprises total ionic impurities at theconcentration of 0.1 ppm or lower and has a volume resistivity of 3×10¹⁰Ωcm or larger.
 12. A light emitting element according to claim 11,wherein the cover member is at least one selected from the groupconsisting of a plastic plate, a glass plate and a PVF film.
 13. Alight-emitting element according to claim 11, wherein the ionicimpurities are sodium or potassium.
 14. A light-emitting elementaccording to claim 11, wherein the plurality of first electrodes arecathodes.
 15. A light-emitting element according to claim 11, whereinthe plurality of second electrodes are anodes.
 16. A light-emittingdevice comprising: a substrate; a plurality of first electrodes arrangedin a stripe shape over the substrate; a light-emitting layer over theplurality of first electrodes; a plurality of second electrodes arrangedin a stripe shape so as to intersect with the plurality of firstelectrodes, over the light-emitting layer; a plurality of color filtersover the plurality of second electrodes; and a cover member over theplurality of color filters, wherein the light-emitting layer comprisestotal ionic impurities at the concentration of 0.01 ppm or lower and hasa volume resistivity of 3×10¹⁰ Ωcm or larger.
 17. A light emittingelement according to claim 16, wherein the cover member is at least oneselected from the group consisting of a plastic plate, a glass plate anda PVF film.
 18. A light-emitting element according to claim 16, whereinthe ionic impurities are sodium or potassium.
 19. A light-emittingelement according to claim 16, wherein the plurality of first electrodesare cathodes.
 20. A light-emitting element according to claim 16,wherein the plurality of second electrodes are anodes.
 21. Alight-emitting device comprising: a substrate; a plurality of firstelectrodes arranged in a stripe shape over the substrate; alight-emitting layer over the plurality of first electrodes; a pluralityof second electrodes arranged in a stripe shape so as to intersect withthe plurality of first electrodes, over the light-emitting layer; aplurality of color filters over the plurality of second electrodes; anda cover member over the plurality of color filters, wherein thelight-emitting layer comprises total ionic impurities at theconcentration of 0.1 ppm or lower and has a volume resistivity of 1×10¹¹to 1×10¹² Ωcm.
 22. A light emitting element according to claim 21,wherein the cover member is at least one selected from the groupconsisting of a plastic plate, a glass plate and a PVF film.
 23. Alight-emitting element according to claim 21, wherein the ionicimpurities are sodium or potassium.
 24. A light-emitting elementaccording to claim 21, wherein the plurality of first electrodes arecathodes.
 25. A light-emitting element according to claim 21, whereinthe plurality of second electrodes are anodes.
 26. A light-emittingdevice comprising: a substrate; a plurality of first electrodes arrangedin a stripe shape over the substrate; a light-emitting layer over theplurality of first electrodes; a plurality of second electrodes arrangedin a stripe shape so as to intersect with the plurality of firstelectrodes, over the light-emitting layer; a plurality of color filtersover the plurality of second electrodes; and a cover member over theplurality of color filters, wherein the light-emitting layer comprisestotal ionic impurities at the concentration of 0.01 ppm or lower and hasa volume resistivity of 1×10¹¹ to 1×10¹² Ωcm.
 27. A light emittingelement according to claim 26, wherein the cover member is at least oneselected from the group consisting of a plastic plate, a glass plate anda PVF film.
 28. A light-emitting element according to claim 26, whereinthe ionic impurities are sodium or potassium.
 29. A light-emittingelement according to claim 26, wherein the plurality of first electrodesare cathodes.
 30. A light-emitting element according to claim 26,wherein the plurality of second electrodes are anodes.
 31. Alight-emitting device comprising: a substrate; a plurality of firstelectrodes arranged in a stripe shape over the substrate; alight-emitting layer over the plurality of first electrodes; a pluralityof second electrodes arranged in a stripe shape so as to intersect withthe plurality of first electrodes, over the light-emitting layer; aninsulating film over the plurality of second electrodes; an organicliquid filler agent over the insulating film; a plurality of colorfilters over the organic liquid filler agent; and a cover member overthe plurality of color filters, wherein a space between the insulatingfilm and the plurality of color filters is filled with the organicliquid filler agent.
 32. A light emitting element according to claim 31,wherein the cover member is at least one selected from the groupconsisting of a plastic plate, a glass plate and a PVF film.
 33. Alight-emitting element according to claim 31, wherein the plurality offirst electrodes are cathodes.
 34. A light-emitting element according toclaim 31, wherein the plurality of second electrodes are anodes.
 35. Alight-emitting element according to claim 31, further comprising asealing member for bonding the substrate and the cover member.
 36. Alight-emitting element according to claim 31, wherein a drying agent isadded to the organic liquid filler agent.
 37. A light-emitting devicecomprising: a substrate; a plurality of first electrodes arranged in astripe shape over the substrate; a light-emitting layer over theplurality of first electrodes; a plurality of second electrodes arrangedin a stripe shape so as to intersect with the plurality of firstelectrodes, over the light-emitting layer; an insulating film over theplurality of second electrodes; a plurality of color filters over theinsulating film; and a cover member over the plurality of color filters,wherein the light-emitting layer comprises sodium at the concentrationof 7×10¹⁷ atoms/cm³ or lower, and wherein a space between the insulatingfilm and the plurality of color filters is filled with an organic liquidfiller agent.
 38. A light emitting element according to claim 37,wherein the cover member is at least one selected from the groupconsisting of a plastic plate, a glass plate and a PVF film.
 39. Alight-emitting element according to claim 37, wherein the ionicimpurities are sodium or potassium.
 40. A light-emitting elementaccording to claim 37, wherein the plurality of first electrodes arecathodes.
 41. A light-emiting element according to claim 37, wherein theplurality of second electrodes are anodes.
 42. A light-emitting elementaccording to claim 37, wherein the organic liquid filler agent includesa drying agent.